Created the TileSheetManager class

This class will manage the large(ish) number of tile sheets for a specific
map.
Revised TileSheet & TileSheetManager
Checking in the test scene and test tilesets
RegionPager now uses a reference to TileSheetManager
The editor scene now works correctly with the TileSheetManager
Slight refactoring
Dummied out the load & save functions